In a crash test environment it is very difficult to measure the velocity between two surfaces at the point or in the proximity of contact. A conventional accelerometer, for example, may not withstand a side impact test due to the close proximity of the test dummy to the reference surface. Another problem with measuring velocities in limited space is that the instrumentation itself may change the characteristics of the crash object and thus affect the crash data.
It would therefore be desirable to measure movement of an object with respect to a surface without changing the characteristics of the surface during the crash, especially, in testing side impact crashes.